In recent years, physical properties of a liquid or molten material are utilized in various fields. For example, in data storage and accumulation fields, such as an optical disk and a phase-change memory, the material is used that is obtained by heating a raw material to thereby melt and then solidify it. In a refining process, the material is directly melted at high temperature, to thereby manufacture a steel product and the like. Further, a liquid-state material, such as liquid crystal, is also used, to constitute a display surface of a image (graphic) display device and the like.
As described above, the physical properties of a liquid or molten material have important factors and functions in various fields. However, the measurement of the physical properties of a liquid or molten-state material is not conducted so often currently because the measurement involves various difficult disadvantages. For example, in the case where the material is in a molten state, the surface of the liquid material that is subjected to the surrounding mechanical vibration, is ruffled. In other words, the liquid material is exposed to an inappropriate condition even under an optical measurement having a characteristic of a noncontact and nondestructive measurement.
In particular, in the case of a material at high-temperature, since it is required to avoid the damage of a measurement instrument due to the high temperature, it is difficult to measure the physical properties by bringing the to-be-measured material to contact with a general sensor or probe. In contrast with this, an optical measurement enables a noncontact and nondestructive measurement. It is possible to measure an object at a distance where no influence by high temperature of the object is caused, because light can be used in the optical measurement. Thus, the optical measurement may be appropriately used in measurement at high temperature.
As an apparatus using this characteristic of the optical measurement, an apparatus is reported in Journal of Non-Crystalline Solids, vol. 315 (2003) 54. According to this apparatus, monochromatic ellipsometry is combined with a heating furnace, to conduct an optical constant measurement of a high-temperature material.
An optical measurement using ellipsometry will be described below. The ellipsometry is a measurement apparatus using polarization of light. The ellipsometry may be simply described as follows. When a certain linear polarization is caused to enter a material to be measured (which is also called “to-be-measured material”) and is reflected from the material, the linear polarization is converted to elliptic polarization. Then, the phase of the elliptic polarization and the polarization degree are measured. Since those reflect the optical information of the to-be-measured material, the thus-reflected light can be analyzed, to determine an optical constant.
The monochromatic ellipsometry is a measurement to use monochromatic light to measure an optical constant, i.e. a refractive index. Generally, He—Ne laser is used, whose wavelength is 632·8 nm. In this case, the optical constant of the molten material only to this wavelength is measured. In contrast with this, spectroscopic ellipsometry using various wavelengths can provide a measurement of light in a wide range from ultraviolet light (wavelength of 250 nm) to far-infrared light (wavelength of 30 μm).